The present technology relates to a solid-state image sensor, a method for the same, and an electronic device, and particularly relates to a solid-state image sensor, a method for the same, and an electronic device which make it possible to reduce occurrence of an after-image caused by charge overflow.
A CMOS image sensor, for example, is used as a solid-state imaging device including a solid-state image sensor. The CMOS image sensor reads out, through MOS transistors, a photocharge accumulated in a pn junction capacitor of a photodiode which is a photoelectric transducer. Since the CMOS image sensor executes an operation of reading out the photocharge accumulated in the photodiode in pixel unit, row unit, or the like, it is not possible to provide all of pixels with the same exposure time period during which the photocharge is accumulated. Accordingly, in such a case that a subject is moving, a captured image might have distortion.
Each of unit pixels of the CMOS image sensor includes a photodiode, a transfer gate, a floating diffusion (FD), a reset transistor, an amplification transistor, and a select transistor.
In this unit pixel, the photodiode is a buried photodiode, for example, by forming a P-type layer in a p-type well layer formed on an N-type substrate and by burying an N-type buried layer (N) in the p-type well layer. The transfer gate transfers the charge accumulated in the pn junction of the photodiode to the floating diffusion (FD).
A mechanical shutter method using a mechanical light shielding unit is widely used as one of methods by which global exposure is implemented for a solid-state image sensor. In the global exposure, an image is captured with all the pixels having the same exposure time period. This mechanical light shielding enables the global exposure to be executed in such a manner that all the pixels simultaneously start exposure and simultaneously terminate the exposure.
In the mechanical shutter method, the exposure time is mechanically controlled to thereby provide each pixel with the same time period in which a charge is generated when light enters the photo diode. Then, the mechanical shutter is closed, and the state changes to a state where no photocharge is substantially generated. In this state, signals are read out sequentially.
However, since the mechanical shutter needs the mechanical light shielding unit, it makes downsizing difficult, and the speed at which the mechanism is driven is limited. For this reason, the mechanical shutter method is inferior to an electrical method in concurrency.
Hence, the electrical global exposure is employed. In the electrical global exposure method, a charge discharging operation is firstly executed simultaneously in each pixel, and then the exposure is started. In the charge discharging operation, the buried photodiode is evacuated with the charge accumulated therein. Thereby, the photocharge is accumulated in the pn junction capacitor of the photodiode.
At the time point when the exposure time period ends, the transfer gate is turned ON simultaneously in each pixel to transfer every accumulated photocharge to the floating diffusion (capacitor).
By closing the transfer gate, the photocharge accumulated in the same exposure time period for all the pixels is held in the floating diffusion.
Thereafter, a signal level is sequentially read out to a vertical signal line, and then the floating diffusion is reset to read out a reset level to the vertical signal line.
When the signal indicating the signal reset level is read out, noise of the signal level is removed by using the reset level in signal processing in the subsequent stage (for example, refer to JP H01-243675A or JP 2004-140149A).
In the noise removing processing, the reset level is read out which results from reset operation executed after the signal level is read out. Accordingly, it is not possible to remove kTC noise (thermal noise) in the reset operation, and thus the image quality is deteriorated.
The kTC noise in the reset operation is random noise generated due to reset transistor switching operation in the reset operation. Accordingly, if a level before transferring the charge to the floating diffusion is not used, it is not possible to appropriately remove the noise of the signal level.
Since the charge is transferred to the floating diffusion simultaneously in each pixel, and thus the noise is removed at this time in such a manner that the signal level is read out and thereafter the reset operation is executed again. Accordingly, it is possible to remove noise such as an offset error, but not possible to remove the kTC noise.
As a technique of enabling the aforementioned kTC noise removal, a unit pixel is proposed which, for example, includes a memory part in the pixel besides a floating diffusion. The memory part is designed to temporarily hold the photocharge accumulated in a buried photodiode. The unit pixel is further provided with a transfer gate which transfers the photocharge accumulated in the photodiode to the memory part.
In executing global exposure in unit pixels each including a memory part, an OFG which resets a PD simultaneously in each pixel is firstly turned ON to execute an operation of discharging the charge in the PD.
Thereafter, the OFG is turned off to start the simultaneous exposure, and thereby the generated photocharge is accumulated in the PD.
Upon completion of the exposure, the transfer gate is driven simultaneously in each pixel to transfer the photocharge to the memory part, and then the transfer gate is turned off to hold the charge in the memory part.
After the transfer gate is turned off, the OFG is turned on to prevent a signal from overflowing from the PD into the memory part holding the charge (blooming) and to perform PD resetting for the next frame.
Thereafter, sequential operations are performed to read out a reset level and a signal level from the charge held in the memory part.
The floating diffusion (FD) is firstly reset, and then the reset level is read out.
Subsequently, the charge held in the memory part is transferred to the floating diffusion (FD) to read out the signal level. At this time, reset noise included in the signal level coincides with reset noise read out when the reset level is read out. This enables processing of reducing noise including even the kTC noise.
That is, according to a pixel structure in which a memory part temporarily holding a charge accumulated in a buried photo diode is provided besides a floating diffusion region, it is possible to achieve processing of reducing noise including even kTC noise.
Examples of an image sensor having such a configuration include JP 2004-111590A and JP 2009-278241A.